No one can “work off” sin. It’s not as if we can “make up for” the injustices we commit against God. As a matter of justice, he deserves all our love and obedience. If we do not love him enough or disobey him, we can never make up for it. Thankfully, God loves us enough that he makes up for it for us (after all, sin does not harm God but it does harm our relationship with him and thus our happiness). Even after sins are forgiven, however, the consequences of them can remain.

The Catechism says:

1472 To understand this doctrine and practice of the Church, it is necessary to understand that sin has a double consequence. Grave sin deprives us of communion with God and therefore makes us incapable of eternal life, the privation of which is called the “eternal punishment” of sin. On the other hand every sin, even venial, entails an unhealthy attachment to creatures, which must be purified either here on earth, or after death in the state called Purgatory. This purification frees one from what is called the “temporal punishment” of sin. These two punishments must not be conceived of as a kind of vengeance inflicted by God from without, but as following from the very nature of sin. A conversion which proceeds from a fervent charity can attain the complete purification of the sinner in such a way that no punishment would remain.

Regarding “working off sin”, that’s not completely correct, but it’s not completely incorrect either. We certainly cannot “work off” the guilt for our sins. We must be forgiven of that guilt by God. But, we can, to one degree or another, “work off” the corrupting effects of our sin upon us. We can work towards sanctification, through prayer and works of love. Nothing unholy can enter heaven, and we can (should, must) work towards holiness in this life.

Never forgetting, of course, that without grace we can do nothing. That much is always a given.

Regarding “working off sin”, that’s not completely correct, but it’s not completely incorrect either. We certainly cannot “work off” the guilt for our sins. We must be forgiven of that guilt by God. But, we can, to one degree or another, “work off” the corrupting effects of our sin upon us. We can work towards sanctification, through prayer and works of love. Nothing unholy can enter heaven, and we can (should, must) work towards holiness in this life. Never forgetting, of course, that without grace we can do nothing. That much is always a given.

I am assuming by guilt you mean culpability and not some vague feeling of being uncomfortable with what we have done. In Catholic Theology serious Sin has consequences, the first of which is the loss of Sanctifying Grace, which may be defined as an ability given to us by God to share in the very life of the Trinity itself(to be eventually in Heaven. Like anyone who breaks a relationship certain temporal consequences occur to us. In any normal human relationship after such a break we may be forgiven, but there still remains some “patching up” to do. This also applies to ones relationship with the Lord. All is forgiven and the sanctifying grace or the relationship is re-established. However some “Patching up” needs to take place. God may not be scarred by the experience, but we are. Some can be accomplished during ones lifetime on earth, but what remains un-patched in the relationship is taken care of in purgatory.